1. Field of the Invention
The present invention is directed to a dual chamber implantable pacemaker including a stimulation pulse generator which generates and emits stimulation pulses to an atrium via an atrial electrode lead and to a ventricle via a ventricular electrode lead, a detector which senses events in the ventricle and a control device which controls the stimulation pulse generator and the detector based on ventricular events detected (or not detected) within specified time intervals and a method for operating such a dual chamber pacemaker.
2. Description of the Prior Art
A dual chamber pacemaker of the type generally described above is disclosed in U.S. Pat. No. 4,825,870 wherein the control device inhibits detector sensing, after an atrial stimulation pulse has been emitted, for a preset blanking interval, orders a ventricular stimulation pulse after a lapse of a preset first A-V interval if no ventricular event is sensed between the end of the blanking interval and the end of the first A-V interval, orders emission of a ventricular stimulation pulse after a lapse of a preset second A-V interval, which is shorter than the first A-V interval, if at least one ventricular event is sensed in a preset crosstalk interval after the blanking interval, and inhibits emission of the ventricular stimulation pulse if a ventricular event is sensed after the lapse of the crosstalk interval.
This known pacemaker is capable of stimulating and sensing both in the atrium and ventricle and only stimulates when needed, i.e., when the heart itself is unable to maintain a normal rate. After every atrial event (stimulation or sensed spontaneous contraction), sensing of the ventricle is inhibited for one blanking interval, also referred to as an absolute refractory period, to prevent signals from the atrium from being interpreted as a ventricular contraction through crosstalk picked up by the detector in the ventricle. Numerous sources can give rise to crosstalk, but crosstalk from atrial stimulation pulses in particular can cause problems. The blanking interval is followed by a crosstalk interval, also referred to as a relative refractory period, during which ventricular events are sensed. However, it is impossible to determine whether events in the crosstalk interval are caused by noise or by a premature ventricular contraction (PVC). Since the sum of the blanking interval plus the crosstalk interval is selected to be less than the natural atrium-to-ventricle conduction time, i.e., the A-V interval, events in the crosstalk interval are interpreted as noise or crosstalk. As a safety measure for instances in which a PVC occurs in the crosstalk interval, the A-V interval programmed in the pacemaker is shortened. This is to prevent stimulation during the ventricle's vulnerable phase in conjunction with repolarization of cardiac tissue. In any event, the ventricular stimulation pulse is inhibited if a ventricular event occurs between the end of the crosstalk interval and the end of the programmed, or shortened, A-V interval.
One problem concerns deciding on the length of the blanking interval. The blanking interval should preferably be long enough to permit interference from the atrial stimulation pulse to abate. If it is too long, however, there is a risk that a PVC might not be detected, and a ventricular stimulation pulse could be emitted during the vulnerable phase, leading at worst to the triggering of fibrillation. On the other hand, a blanking interval which is too short often leads to detection of noise in the crosstalk interval. Ventricular stimulation with a shortened A-V interval could accordingly occur more frequently. As a result of the shorter A-V interval, a natural ventricular contraction might not have time to occur before the stimulation pulse is emitted. Unnecessary stimulation could thus occur, causing a needless energy drain on the implantable pacemaker's battery.
A pacemaker is disclosed in U.S. Pat. No. 4,974,589 in which the blanking interval can be initiated on multiple occasions during a single cardiac cycle. A first blanking interval is instituted after an atrial stimulation pulse, and a first crosstalk interval follows at the end of that blanking interval. If an event is detected within a given part of the first crosstalk interval, a second blanking interval is commenced. A second crosstalk interval starts thereafter, and a third blanking interval starts if an event is sensed within that given period. This sequence of blanking and crosstalk intervals continues until no event is sensed in the given part of the respective crosstalk interval, whereupon the crosstalk interval continues in the same way as in the pacemaker described in the aforementioned U.S. Pat. No. 4,825,870 or until a maximum period of time elapses for blanking and crosstalk intervals, all subsequent events then being interpreted as ventricular contractions.
Even if this solution does produce immediate adaptation of the blanking interval, it does not fully solve the problem of minimizing the duration of the blanking interval without emission of excessive numbers of ventricular stimulation pulses with shortened A-V intervals. In addition, a PVC could appear in one of the crosstalk or blanking intervals without being registered as anything other than noise.